Fiber optic transmission systems transmit information from one location to another by sending optical energy through glass fiber. Information is encoded on the energy by modulating either its power or spectrum. In these systems it is often necessary to monitor the average optical power being carried by a transmission fiber. Average optical power is a useful indicator of system performance, and thus is often used for maintenance and control purposes.
Known means for measuring optical power in a fiber use one of two methods. The simplest method is to break the optical path by disconnecting one end of an active fiber from a system. Preferably this is done at a site where the fiber is connectorized and thus easily removable. The disconnected fiber end is inserted into an optical power meter. Optical energy in the fiber under test passes out of the fiber end and enters the power meter for measurement. After a measurement is completed, the fiber is then reconnected to the system.
While this method is simple, the requirement of having to break the optical path and thus interrupt system operation during measurement is a severe limitation in commercial systems carrying traffic. In addition, the need to disconnect and then reconnect the test fiber each time a measurement is made can degrade the optical connection between test fiber and system, resulting in a loss of optical energy from the system.
A second known method for measuring optical power carried by a fiber avoids having to disrupt system operation during measurement by inserting a fiber optic tap into the optical path. The fiber optic tap extracts a small fraction of optical power from the optical path and delivers it to a power meter for measurement. Since most of the optical energy passes through the tap undisturbed, the transmission system under test continues to operate normally while a measurement is made. The ratio of optical power removed by the tap to the optical power remaining in the test fiber is defined as the tap ratio. Because this ratio is fixed, optical power removed by the tap can be used as a measure of optical power carried by the test fiber.
Known fiber optic taps suitable for use in this measurement method include multi-port devices such as fused fiber couplers (see for example U.S. Pat. No. 5,251,277 issued to D. Young on Oct. 5, 1993), and temporary taps made by bending the transmission fiber to scatter out optical energy. An example of the latter type of tap is described in U.S. Pat. No. 5,127,724 issued to S. James, D. Ferguson, D. Drouet, S. Hornung on Jul. 7, 1992.
Measurement of optical power using the above mentioned optical taps suffer from several important limitations. For example, fiber optic devices that serve as taps are by necessity three-port devices (see for example, Chapter 26, "Photonic Local Networks", by I. P. Kaminow of Optical Fiber Telecommunications II, Edited by S. E. Miller, and I. P. Kaminow, Academic Press, Inc. Boston, Mass. 1988, pages 940-943). Optical energy must enter the device through an input port and exit through two output ports. Performing a single power measurement thus requires an operator to make three optical connections, each requiring special tools and skill on the part of the operator. The resulting time required to perform each measurement is undesirable when large numbers of fibers need to be measured.
The use of a temporary tap, such as a fiber bend, avoids this problem since the optical tap can be introduced and removed easily. However the temporary tap suffers from poor accuracy owing to a dependence of tap ratio on fiber waveguide design and coating parameters. In addition, a temporary tap is not applicable to cabled fiber since the light scattered out of the fiber can not penetrate the cable jacket.
In addition to the above mentioned limitations, presently known means for measuring optical power in fibers suffer from an additional limitation. Because transmission fibers are often located in the field or widely scattered over a given location, access to commercial electrical power is often difficult. As a result it is often necessary to use portable battery-powered meters to measure optical power. However, known portable meters typically have battery life of less than 100 hours when operated continuously. This makes these meters impractical for long term monitoring of fiber optic transmission lines owing to the need for frequent battery replacement.
Although battery life in known portable meters can be extended by increasing the size of batteries used, this limits the portability of the meters. It is the trade-off between portability and battery life in known portable meters that limits their range of application. For example, the feasibility of integrating a power meter into a fiber optic cable is determined by the maximum weight that the cable and fiber optic connectors can support with out affecting optical loss. For many commercially available fiber optic connectors this maximum weight is as low as 120 grams-weight. Known portable meters exceed this weight limit and thus are not suitable for this application.